It's so you can post packages and then immediately use them, or tell someone else how to immediately use them, by telling
mmc-get to use the unreviewed list. And, it's so that people can by default use a reviewed list that definitely won't have spam or other abuse.
There are other ways to control abuse but not many that work well with my "no credentials at all" strategy on adding packages.
It's perfectly easy for third-party tools that are written in Mercury, though.
If the type changes, you'll start getting errors and so will know immediately :)
You can do this locally with
mmc-get submit (when that's implemented) or you can use this form
If the meaning's unclear and the package has been reviewed, it might be on this table
Submit a package with dummy values and a statement of your distaste in the comment field. Comments in unreviewed packages should be directed at the package maintainer, and comments in reviewed packages will be comments from the package maintainer (generally they'll just be stripped).